The trailing edges of turbine blades for gas turbine engines are often cooled using an impingement heat transfer system. The impingement system works by accelerating a flow through an orifice and then directing this flow onto a downstream surface to impinge upon a desired heat transfer surface. When applied to the trailing edge of a cooled turbine airfoil, the system typically assumes the form of a group of crossover holes in one or more ribs. Cooling flow is accelerated from the upstream cavity, which is maintained at high pressure on one side of the rib to the impingement cavity, which is maintained at lower pressure on the other side of the rib. An example of such a trailing edge impingement cooling system is depicted in FIGS. 1 and 2. In this particular example, two impingement cooling systems are employed in a series arrangement. As shown in FIG. 1, a turbine blade indicated generally by the reference number 10 defines a first feed cavity 12 and a second feed cavity 14 connected in series. The second feed cavity 14 communicates with first and second transition chambers 16, 18 defined by the blade 10 at a transition region to supply an impinging jet of a cooling medium through the transition chambers and to an ejection slot 22 defined by the blade at a trailing edge region 24 thereof. The overall impingement cooling system can include any arrangement of independent impingement cooling systems or multiples thereof combined in series or in parallel with one another.
The impingement cooling system facilitates cooling of the trailing edge region 24 by promoting convective heat transfer between the cooling medium and the internal walls of the component. Convective cooling is promoted both within the impingement cavity itself and also within impingement holes.
In the typical trailing edge impingement cooling system, a set of impingement holes is typically centered along a central longitudinal axis of a set of impingement ribs defining the impingement holes. This is due, in part, to perceived constraints of the investment casting process, which is used to fabricate the part, and also to focus the impinged flow on a particular downstream target surface. With the impingement holes located centrally within the impingement ribs, the propensity to cool the concave and convex surfaces of the airfoil via convection into the impingement holes are relatively consistent because the conductive resistances are essentially the same in either direction.
As best shown in FIG. 2, the turbine blade 10 including a conventional trailing edge impingement system has a first set of impingement holes 26 defined by impingement ribs coupling the second feed cavity 14 and the first transition chamber 16, and a second set of impingement holes 28 defined by impingement ribs coupling the first transition chamber 16 and the second transition chamber 18. As shown in FIG. 2, the impingement holes 26, 28 each have a central longitudinal axis extending in a direction of airflow which generally coincides with a localized central longitudinal axis of the impingement ribs or of blade 10. In other words, the first and second sets of impingement holes 26, 28 each have a central longitudinal axis which is generally equidistant from a nearest portion of an edge 30 of the blade at a convex side 31 and a nearest portion of an edge 32 of the blade at a concave side 33. As a result, a conduction resistance 34 on a concave side of the blade 10 is generally equal to a conduction resistance 36 on a convex side of the blade.
The problem with prior trailing edge impingement cooling systems involves cooling of the airfoil concave and convex sides by impinging jets of a cooling medium when the heating from the two sides is substantially unequal. For example, the heat load imposed on the concave (pressure) side of an airfoil can be much greater than that imposed in the convex (suction) side because of the influences of accelerating flows, roughness and deleterious film cooling effects such as accelerated film decay characteristics on the concave side.
Accordingly, it is an object of the present invention to provide a trailing edge impingement cooling system for a turbine blade of a gas turbine engine that overcomes the above-mentioned drawbacks and disadvantages.